Field of the Invention
The present invention relates to an image forming method and apparatus for printing out, e.g., image data output from a scanner and image data output from an external apparatus and, more particularly, to an apparatus for printing out color image data.
As a color image forming apparatus for printing out image data output from, e.g., a scanner, a computer, or the like, a color copying machine has been conventionally proposed.
Colors to be finally printed by a color copying machine are four colors, i.e., C, M, Y, and K, and the color copying machine performs signal processing for converting R, G, and B image data output from a scanner or an external apparatus such as a computer into C, M, Y, and K data.
However, the conventional color copying machine suffers the following problems.
Upon conversion from R, G, and B.fwdarw.C, M, Y, and K (LOG conversion, black extraction, output masking, UCR), when the UCR amount is large, the ratio of K becomes high as compared to the amounts of C, M, and Y on a gray (white/black) portion of an image. Especially on a highlight gray portion (light gray portion) in a "normal copy mode", the noise amount increases due to light shot noise on a CCD (which noise is proportional to a square of the amount of light incident on the CCD and becomes larger as the gray portion is lighter), and this noise is included in a K signal having the highest ratio of C, M, Y, and K data by black extraction and output masking.multidot.UCR processing. As a result, "granularity" of an image becomes conspicuous.
It is empirically known that the "granularity" of an image can be eliminated by changing the coefficients used in the output masking.multidot.UCR processing to decrease the ratio of the K signal and to increase the ratios of the C, M, and Y signals. However, with this processing, when C. G. image data (including no noise) created by a computer is to be printed out via an external I/F, or when gray characters (character document or characters included in a map) are to be printed out even in a normal copy mode, the C, M, and Y colors on a gray (white/black) portion are recorded at relatively higher ratios, and the gray portion is printed in gray with a color appearance, thus impairing gray reproduction upon printing of a C. G. image.
As a method of extracting a K signal from C, M, and Y signals, a scheme designed for optimizing K data in a C. G. image, a character document, and a character portion of a map document (further improving gray reproducibility) is available (to be described in detail in the embodiment of the present invention). However, such a new black extraction method has a nature of emphasizing a noise component in a normal natural image, thus posing another problem.